There is a known technique to convert an instruction value into a 1-bit pulse string using a ΔΣ modulator in order to obtain a signal for controlling switches included in a main circuit of an inverter circuit.
In this technique, an instruction value u(t) is converted into a 1-bit pulse string v(t) by a ΔΣ modulator and the 1-bit pulse string v(t) is used for switching control of the switches included in the main circuit. By this kind of control, an alternate current i(t) can be generated from a DC voltage Vdc. The alternate current i(t) is used for driving a motor M, for example.
When the instruction value has a sine wave, its spectrum becomes a single spectrum. On the other hand, a spectrum of a quantization error (quantization noise) generated at a quantizer becomes a uniformly-distributed continuous spectrum with no specific peak. Therefore, an inverter circuit using a ΔΣ modulator has a feature in that EMI (Electro-Magnetic Interference) rarely occurs.
A voltage-output inverter circuit, such as a power conditioner, smooths a waveform of an AC voltage generated by a main circuit by removing high-frequency components included in the AC voltage, through an LC filter having a reactor (L) and a capacitor (C) provided between the main circuit and a load.
The frequency characteristic of an LC filter has 0 dB in a low frequency range, attenuation characteristic in a high frequency range, and a peak at an LC oscillation frequency wo. Therefore, a wo component of a quantization noise included in an AC voltage output from a main circuit is inevitably amplified. Due to the amplified wo component, the output voltage of the LC filter is superimposed with fluctuations of about 20 kHz that is the LC resonant frequency, in addition to a 50-Hz sine wave, resulting in having a waveform completely different from a sine wave. Especially, the frequency spectrum of this output voltage has a high peak at an LC resonant frequency f0=wo/2π.
As discussed above, a known inverter circuit using a ΔΣ modulator has a problem in that a sinusoidal voltage cannot be output when an LC filter is connected to a main circuit.